1. Field of the Invention
The invention relates to a multiplex terminal station apparatus for use in an optical communications system and, more particularly, to a multiplex terminal station apparatus having a duplex-system configuration to maintain reliability.
2. Description of the Related Art
An optical communications system with high-reliability requirements often uses a duplex-system configuration for optical terminal station apparatuses and transmission links for the purpose of preventing the system from going down due to failures in various pieces of equipment and transmission links used in the system.
When such a duplex-system configuration is used, a conventional system allows the service system (the currently used system) and the protection system to transmit the same information.
However, if the service system is normal, information transmitted by the protection system will be discarded. That is, if the service system is normal, the protection system is not practically used. Thus, the protection system is completely useless as long as the service system is normal.